Range finding by diffraction is comprised of the methods, devices and systems used to measure distance through exploitation of a phenomenon observed with diffraction gratings wherein the displacement between diffraction images of the various diffraction orders can be correlated to the distance from the grating to an observed source of energy illuminating the grating. Higher-order diffraction images of a target are reconstructed at a receiver which has a means to focus the radiation onto a transducer that can sense the position of the higher-order diffraction images. As a target is moved toward or away from a grating surface, the relative displacement of a higher-order image from both the zero-order image and other higher-orders images can be measured so as to take target range. The present inventor has demonstrated such a range finder under grants from the National Science Foundation (NSF DMI-9420321). When the diffraction grating is the hologram of a point source and the target is positioned at an angle of grazing incidence relative to the grating, it is possible to form profiles in the microscopic regime. Such an embodiment was developed under a grant from the National Science Foundation (NSF IIP-0724428).
The basis of the present invention derives from U.S. Pat. No. 6,490,028, “VARIABLE PITCH GRATING FOR DIFFRACTION RANGE FINDING SYSTEM,” issued to Ditto and Lyon on Dec. 3, 2002 (hereinafter, '028 patent). As illustrated by FIG. 4 (a) in the '028 patent supra, and reproduced here as FIG. 1 in accordance with the prior art, a lens 210 is employed to focus higher-order diffraction images inside camera 200. Exemplary rays are traced from range points 330 on light beam 320 along rays 160 through variable pitch diffraction grating 122 after which ray bundles 150 are brought to a focus inside camera 200. The rays cross through a point at a perspective center inside lens 210. This point is a pinhole approximation of a lens. When an actual lens is used, alternative ray paths in bundles 160 and 150 will result in a less than optimal focus at the receiver, most particularly when the grating 122 is a variable pitch grating (also called a “chirped grating”).
The utility of a pinhole at lens 210 of FIG. 1 can be appreciated by an understanding of the fabrication of the variable pitch grating itself. The variable pitch grating used in the '028 Patent supra can be fabricated by means of holography. A variable pitch hologram can be created through the intersection of a plane wave originating from a collimator and a spherical wave originating from the pinhole aperture in a spatial filter, a process that is cited in '028 Patent supra by reference to U.S. Pat. No. 3,578,845 issued to Brooks et al. on May 18, 1971 for “Holographic Focusing Diffraction Gratings for Spectroscopes and Method of Making Same.”
The holographic optical train can be a recording process of the type illustrated in FIG. 2, in accordance with the prior art. Laser 400 produces a coherent monochromatic collimated beam of light 401 which is divided by beam splitter 411 into two beams 402. Spatial filter 412, comprised of a combination lens and pinhole, expands one beam 402 into a spherical wave 403 which is collimated by parabolic mirror 413 and made incident as wavefront 404 upon holographic recording plate 416 set at angle i relative to incident plane wave 404. The other laser beam 402 divided by beam splitter 411 is also sent by folding mirrors 414 to spatial filter 415 where it is expanded into a spherical wave 405 to be incident at the surface about normal to holographic plate 416. The wavefronts 404 and 405 interfere to cause a pattern that constitutes the variable pitch grating used in a diffraction range finder.
When a pinhole is used in lieu of lens 210 in FIG. 1 as per the illustrations of '028 Patent supra, the images formed in camera 200 are sharply focused. If the pinhole is of exactly the same diameter as the pinhole that was used to make the spherical wave in the fabrication of the hologram, i.e., the variable pitch grating, the resulting image formed on the image plane of the camera can be optimal in acuity. However, very little light is captured by the camera from the point of origination along the light beam 320 projected from laser 300, because of the small dimensions of the pinhole. Alternatively, a lens 210 can be used in front of the camera 200, but when a normal lens is used, multiple ray paths through the lens create a focus blur in the final image.